1. Field of the Invention
This invention relates broadly to a Stirling engine. More particularly, this invention relates to a cam drive system for a Stirling engine that converts linear mechanical motion of pistons into rotary motion at an output shaft and vice versa.
2. State of the Art
Stirling engines are heat engines that operate on a closed thermodynamic cycle to convert heat energy into mechanical energy by alternately compressing and expanding a confined working fluid (gas or liquid). As with any heat engine, the engine requires a hot sink and a cold sink and, in the Stirling engine, the confined working fluid is externally heated and cooled. Unlike a steam engine, the working fluid does not change phase at anytime during the thermodynamic cycle. The alternate heating and cooling of the working fluid produces an alternating pressure within the engine. The alternating pressure (or pressure wave) can be converted to mechanical power by several means. For example, the pressure wave can act on pistons, bellows, or diaphragms to convert the pressure wave into mechanical power. Pistons, bellows, and diaphragms produce linear motion that must be converted to rotary motion where rotary motion engine output is desired.
There are a number ways to accomplish the conversion of linear motion from the piston into rotary motion. Crankshafts, wobble-plates, swash-plates, cams, and various other means have been used in the past.
Theories claim Stirling engine performance can be improved by causing a displacer of the engine to dwell at top dead center and bottom dead center. By dwelling at these positions, the working fluid remains in a heat exchanger of the engine for a longer time resulting in greater energy transfer to or from the walls of the heat exchanger to or from the working fluid. Dwells in motion are relatively easy with cams as compared with other mechanisms such as cranks, wobble-plates, and swash-plates that inherently produce sinusoidal or nearly sinusoidal motion. The cam followers engaging the cams can either be sliding or rolling.
High-speed cam design requires attention to the first three derivatives of the displacement function: velocity, acceleration, and jerk. The displacement required is defined by the piston stroke. The shape of the cam curve with respect to rotation is made up of intervals of rise, fall, and dwell. During dwell, there is no piston motion as the cam rotates. The intervals are designed and pieced together so that there preferably are never infinite or excessively high values of acceleration and/or jerk. By controlling acceleration and jerk, the forces on a cam follower and associated moving components can be kept to acceptable levels. This also reduces wear, spalling, and friction on the followers and cam surface in contact with the follower.
Except at very low speeds, sliding cam followers require copious lubrication to maintain a hydrodynamic barrier between the follower and cam surface. Lubrication can be achieved by submersion or pump flooding the cam/follower contact area. The follower rides on a thin hydrodynamic layer of lubricant that reduces friction, prevents high speed contact, and carries away heat that may be generated. However, at high speeds, sliding cam followers require a crankcase containing a fluid lubricant such as oil or grease (wet sump).
Rolling followers can also be used, but have other problems. U.S. Pat. No. 4,996,953 to Buck describes a grooved cam system for a Stirling engine. When the direction of follower load reverses, as it will with double-acting Siemens-type Stirling pistons, the cam follower alternately contacts both sides of the cam groove as the cam rotates. Because the cam rotates in one direction continuously, the rolling follower must reverse direction instantly when switching contact from an upper surface to a lower surface. This reversing may be acceptable for small light weight follower bearings operating at low speeds but large heavy follower bearings rotating at high speeds have considerable inertia and attempting to instantly reverse direction when contacting the opposite surface results in skidding and destruction of the mating follower and cam surfaces.
U.S. Pat. No. 3,385,051 to Kelly teaches a dual blade cam system in which each of two wave-shaped blade cams extends radially outward from the output shaft of the engine. Roller bearings are provided on first and second sides of each of the cams. Blade-type cylindrical cams do not have the problems associated with reversing follower direction of rotation, because for reversing follower loads there are two followers, one above the cam blade, and one below. Each follower is continuously in contact with the same cam surface moving in the same direction. Therefore, there is no skidding. However, these follower assemblies tend to be large, heavy, complex, and expensive. Moreover, unless preloaded, these assemblies can be particularly loud, especially when the load reverses directions and the follower in contact with a cam surface is changed.
Some Stirling engines, such as swash-plate drive engines, operate with wet sumps and require sealing at the piston drive rods to prevent oil from entering the working fluid space from the crankcase fluid space as well as containing the working fluid in the working space. Lubricant in the working fluid can contaminate heat exchanger surfaces or plug the fine pores in the regenerator. Contaminated heat exchangers can reduce performance or cause the engine to be inoperable. Contaminated heat exchangers are difficult or impossible to clean. Explosion in the heater can result if the working fluid is air containing oxygen and the contaminating lubricant is flammable. Because of potential contamination or explosion hazard, and the desire to be able to operate in any orientation, dry-sump Stirling engine designs are desirable.
It is therefore an object of the invention to provide an improved cam drive mechanism for the conversion of Stirling engine piston linear motion to output shaft rotary motion and vice-versa.
It is another object or the invention to provide for optional cam shapes to produce various cam follower (thus piston) motions (displacement, velocity, acceleration, and dwell) such that the Stirling thermodynamics may be exploited by using optimized piston motions.
It is also an object of the invention to provide a compact Stirling engine mechanical drive that has low volume and weight with respect to traditional Stirling engines.
It is a further object of the invention to provide a high efficiency (low friction) mechanical drive.
It is an additional object of the invention to provide a drive mechanism that is easily manufactured and thus less costly to produce.
It is yet another object of the invention to provide a drive mechanism that is reliable and has low maintenance requirements.
In accord with these objects, which will be discussed in detail below, a Stirling engine is provided having a grooved cam drive mechanism, with cam followers coupled to each piston of the engine and engaged within the grooved cam. Each follower includes a pair of longitudinally displaced bearings. One bearing is adapted to ride along an upper inner surface of the cam, while the other bearing is adapted to ride along a lower inner surface of the cam.
More particularly, each follower includes an outer shaft on which a first of the bearings is mounted, and an inner shaft on which a second of the bearings is mounted. A preferably annular space is provided between the inner and outer shafts when the follower is in an unloaded state. Then, when the follower is engaged within the grooved cam, the inner shaft is cantilevered relative to outer shaft within the annular space and results in pre-loading the first bearing against one inner surface of the groove cam and the second bearing against an opposite inner surface of the grooved cam. The pre-loading eliminates excessive noise and increased bearing wear that would otherwise result.
In accord with one embodiment of the invention, the axes of rotation for the bearings are offset by a first amount in the unloaded state, and a second lesser amount in the loaded state.
In accord with another embodiment, the cam groove has a stepped surface and the bearings of a cam follower have different diameters but a common rotational axis in the unloaded state. When the follower inserted into the groove, the axes of rotation for the bearings are offset, and the larger diameter bearing bears against a surface opposite the step and the smaller diameter bearing bears against a surface of the step.
The bearings are preferably crowned, i.e., have a preferably spherically curved surface. This permits line contact with the cam surface thus reduces the effect of the difference in cam surface velocity at different radial distances from the output shaft rotation centerline. Moreover, the tandem pair of bearings on each follower provide greater load carrying capacity. Furthermore, each bearing is dedicated to rotation in only a single direction.
The cam and followers of the invention provide an engine capable of operating at high speed and low noise. Furthermore, the cam drive mechanism operates with low wear. Moreover, the cam and followers are easily manufactured, and provide a compact, relatively inexpensive, and light weight assembly.
Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures.